The present invention relates to the area of steels for application in the field of metal containers for food, non-food products or industrial purposes.
The steels smelted for uses specific to metal containers differ from thin sheets in particular by their physical characteristics.
The thicknesses of steel sheets for containers vary from 0.12 mm to 0.25 mm for the great majority of uses, but can reach greater thicknesses, as much as 0.49 mm, for very special applications. This is the case, for example, of certain containers for non-food products, such as certain aerosols, or the case of certain industrial containers. Their thickness can also be as small as 0.08 mm, in the case of food receptacles, for example.
Steel sheets for containers are usually coated with a metal coat (tin, which may or may not be remelted, or chrome), on which there is generally deposited an organic coat (varnish, inks, plastic films).
In the case of two-piece containers, these are made by deep-drawing under a blank holder or by deep-drawing/trimming for beverage cans, and are generally cylindrical or frustoconical, axially symmetric cans. The container designers are showing increasing interest in even thinner steels, however, with thickness from 0.12 mm to 0.075 mm and, with the objective of distinguishing themselves from the competitors, they are trying to introduce increasingly more complex shapes. Thus we now find cans of original shapes, manufactured from steel sheets of small thicknesses, which sheets, even though presenting greater forming difficulties, must meet the use criteria (mechanical durability of the containers, resistance to the axial load to which they are subjected during storage in stacks, resistance to the internal overpressure to which they are subjected during sterilizing heat treatment and to the internal partial vacuum to which they are subjected after cooling) and therefore must have very high mechanical strength.
Thus the use and performance of these containers depend on a certain number of mechanical characteristics of the steel:
coefficient of planar anisotropy, xcex94C aniso,
Lankford coefficient,
yield strength Re,
maximum rupture strength Rm,
elongation A%,
distributed elongation Ag%.
To impart to the container equivalent mechanical strength at smaller steel thickness, it is indispensable that the steel sheet present a higher maximum rupture strength.
It is known that containers can be made by using standard aluminum-killed low-carbon and low-manganese steels.
The carbon content customarily sought for this type of steel ranges between 0.020% and 0.040%, because contents in excess of 0.040% lead to mechanical characteristics less favorable for deep-drawing, and contents below 0.015% bring about a tendency to natural aging of the sheet, despite an aging in annealing.
The manganese is reduced as much as possible because of an unfavorable effect of this element on the value of the Lankford coefficient for steels not degassed under vacuum. Thus the manganese content sought ranges between 0.15 and 0.25%.
These steel sheets are made by cold rolling a hot strip to a cold-rolling ratio of between 75% and more than 90%, followed by continuous annealing at a temperature of between 640 and 700xc2x0 C., and a second cold-rolling with a percentage elongation which varies between 2% and 45% during this second cold-rolling depending on the desired level of maximum rupture strength Rm.
For aluminum-killed low-carbon steels, however, high mechanical characteristics are associated with poor elongation capacity. This poor ductility, apart from the fact that it is unfavorable to forming of the container, leads during such forming to thinning of the walls, a phenomenon which will be unfavorable to the performances of the container.
Thus for example, an aluminum-killed low-carbon steel with a maximum rupture strength Rm on the order of 550 MPa will have a percentage elongation A% on the order of only 1 to 3%.
The objective of the present invention is to provide an aluminum-killed low-carbon steel sheet for containers which has, at a level of maximum rupture strength equivalent to that of aluminum-killed low-carbon steels of the prior art, a higher percentage elongation A%.
To achieve these characteristics, the invention has as its object a process for manufacturing an aluminum-killed low-carbon steel strip for containers in which:
a hot-rolled steel strip is supplied which contains by weight between 0.022 and 0.035% of carbon, between 0.15 and 0.25% of manganese, between 0.040 and 0.070% of aluminum, between 0.0035 and 0.0060% of nitrogen, the remainder being iron and the inevitable trace impurities,
the strip is passed through a first cold-rolling,
the cold-rolled strip is subjected to annealing,
a secondary cold-rolling is performed if necessary,
characterized in that the annealing is a continuous annealing in which the cycle comprises a temperature rise up to a temperature higher than the temperature of onset of pearlitic transformation Ac1, holding the strip above this temperature for a duration of longer than 10 seconds, and rapidly cooling the strip to a temperature of below 350xc2x0 C. at a cooling rate in excess of 100xc2x0 C. per second.
According to other characteristics of the process according to the invention:
the strip is maintained during annealing at a temperature of between Ac1 and 800xc2x0 C. for a duration ranging from 10 seconds to 2 minutes;
the cooling rate is between 100xc2x0 C. and 500xc2x0 C. per second;
the strip is cooled at a rate in excess of 100xc2x0 C. per second to room temperature.
The invention also relates to an aluminum-killed low-carbon steel sheet for containers, comprising by weight between 0.022 and 0.035% of carbon, between 0.15 and 0.25% of manganese, between 0.040% and 0.070% of aluminum, between 0.0035 and 0.0060% of nitrogen, the remainder being iron and the inevitable trace impurities, which steel is manufactured according to the foregoing process, characterized in that it has in the aged condition a percentage elongation A% satisfying the relationship:
(670xe2x88x92Rm)/14xe2x89xa6A%xe2x89xa6(720xe2x88x92Rm)/17
where Rm is the maximum rupture strength of the steel, expressed in MPa.
According to other characteristics of the sheet, the steel contains carbon in free state and/or some carbides precipitated at low temperature, and it has a grain count per mm2 greater than 20000.